There are various functions of coating films formed from coating compositions, and with respect to functions to protect substrates covered with coating films from external factors, such as the mechanical strength, the weather resistance, the water/moisture resistance and the electrical insulating properties, their properties required are becoming severe in recent years.
Particularly, with respect to the moisture resistance to protect substrates from moisture, properties required in the field of backsheets for solar cell modules and the like are becoming severe.
A backsheet for a solar cell module is required to have moisture resistance, and as a backsheet for a solar cell module having moisture resistance, one having a cured coating film of a curable functional group-containing fluorinated polymer coating formed only on one side of a water impermeable sheet has been proposed (Patent Document 1). Such a backsheet for a solar cell module can be used for a crystalline silicon solar cell, but cannot be used for an element of which the efficiency decrease by moisture is remarkable, such as an organic dye sensitized element.
As another method to impart moisture resistance to a coating film, a method of forming a metal layer considered to have high moisture resistance between a substrate and the coating film may also be considered, however, in such a case, a plurality of steps including a step of forming a metal layer on the substrate and a step of further forming the coating film thereon are required, thus leading to complicated production.
On the other hand, heretofore, a coating composition having a scaly leafing aluminum pigment subjected to surface treatment with stearic acid or the like blended has been used. It has been known that by such a coating composition, a coating film to be formed on a substrate is a coating film having a layer in which aluminum pigment particles are aligned in parallel with the substrate at the surface layer portion (vapor phase side) of the coating film regardless of the coating method. By the coating film having a layer in which aluminum pigment particles are aligned, the moisture resistance of the coating film will be improved. However, as the layer in which aluminum pigment particles are aligned is at the surface layer portion of the coating film, a further protective layer will be required to prevent deterioration of aluminum by corrosion or the like.
Further, it has also been known to blend a non-leafing aluminum pigment not subjected to surface treatment with stearic acid or the like with the coating composition. In such a case, although non-leafing aluminum particles will be aligned in parallel with the substrate in the coating film, they are irregularly present in the coating film as different from the leafing aluminum pigment, and accordingly the moisture resistance of the coating film will not be improved. Accordingly, it is necessary to blend a larger amount of the aluminum pigment with the coating composition so as to increase the moisture resistance of the coating film by blending the non-leafing aluminum pigment. However, if a certain amount or more of the aluminum pigment is blended, the absolute amount of the aluminum pigment in the coating film formed on the substrate will be increased, whereby embedding of the aluminum pigment in the coating film components tends to be insufficient, whereby the stability of the aluminum pigment and the strength of the coating film may not be maintained.